The sortase A (SrtA) enzyme from Staphylococcus aureus is widely used as a tool to catalyze in vitro ligation reactions that join biomolecules. It is arguably the most effective tool for this purpose. It functions as a transpeptidase that joins two peptides, a peptide containing the sequence LPXTG (SEQ ID NO:2) (where X can be any amino acid) and a peptide containing a polyglycine (e.g., penta-glycine (Gly5, SEQ ID NO:1)) at its N-terminus. The latter peptide can contain as few as three glycine residues. In the reaction, SrtA joins the peptides by cleaving the LPXTG (SEQ ID NO:2) motif between threonine and glycine and subsequently joins the carboxyl group of threonine to the amino group of penta-glycine (Mazmanian (1999) Science, 285:760-763; Spirig et al. (2011) Mol. Microbiol. 82:1044-1059). As a result the original substrates are joined via peptide bond.
SrtA transpeptidation reaction has been used for generating antibody conjugates, generating nucleic acid-protein fusions, PEGylating and/or lipidating proteins, in live cell-labeling, in protein cyclization, in silent labeling, in domain labeling, and to add proteins to solid supports (see, e.g., Parthasarathy et al. (2007) Bioconjug. Chem. 18: 469-476; Chan et al. (2007) PLoS One, 2: e1164. doi:10.1371; Pritz et al. (2007) J. Org. Chem. 72: 3909-3912; Antos et al. (2008) J. Am. Chem. Soc. 130: 16338-16343; Kobashigawa et al. (2009) J. Biomol. NMR, 43: 145-150; Sakamoto et al. (2010) Bioconjug. Chem. 21: 2227-2233; Levary et al. (2011) PLoS One, 6: e18342; Refaei et al. (2011) J. Biomol. NMR, 49: 3-7; Freiburger et al. (2015) J. Biomol. NMR, 63: 1-8).